In the manufacture of products based upon polyolefins, it is essential to include suitable additives to inhibit oxidation and prolong the life expectancy of the material. The effectiveness of the antioxidant is, of course, dependent upon the chemical characteristics that it possesses, some types being more effective than others with any given polymer system. The amount used will also influence effectiveness. Of particular significance, however, is the degree of dispersion obtained in the polymer system. For any antioxidant in any given amount, the true effectiveness is directly related to the dispersion achieved. A number of side effects, independent of the basic function of oxygen attack inhibition, are caused by poor dispersion of the antioxidant incorporated. These include, but are not limited to poorer mechanical properties such as reduced tensile and tear strength, reduced resistance to low temperature flexing and lower elongation. Electrically, poor dispersion can vastly reduce breakdown voltage strength of the polyolefin dielectric, and can contribute to electrical losses by increasing power factor and dielectric constant.
There are numerous ways practiced commercially to incorporate antioxidant materials into polymer systems. Method 1. The antioxidant can be added during manufacture of the raw polymer which, while possibly achieving reasonably good dispersion, limits the amount and type incorporated to one entity. Method 2. The most common approach is utilization of high shear mixers. The latter system requires subjecting the base polymer to high shear forces at substantially elevated temperatures for relatively long periods of time. Often due to the very high melting points of most antioxidants, the material being dispersed does not melt and therefore can easily form agglomerates of particles of itself or with other components in a mixed composition. Once agglomerates are formed, it is unlikely that they are broken down completely by the shear forces that are available in the mixing operation. The heat history during high temperature, high shear mixing uses up some of the antioxidant and causes undesired gelled polymer particles to form. Additionally, the high shear forces involved reduces the molecular weight of the polymer system.
The third technique, Method 3, having the advantage of eliminating the high shear forces and most of the heat history associated with conventional mixing operations, is described in U.S. Pat. No. 3,455,752. In this method, the base polymer is polyethylene with or without fillers and other modifying ingredients which is introduced in pellet form at room temperature into a ribbon blender or similar type of mixer having a stirring or tumbling action, and a peroxide type of curing agent is incorporated by diffusion through the pellet wall. This operation is carried out usually with the mixing chamber walls heated but to a temperature below the softening point of the pelleted base material. The highest temperatures in this system to which the polymer is subjected is, therefore, some temperature lower than the temperature of the mixer, but substantially below the softening point of the composition. The peroxide used has a melting point of about 20.degree. to 25.degree. C above room temperature and becomes liquified as batch temperature increases. In this system, antioxidants in very minor amounts, such as 0.1 parts per 100 parts of polyethylene pellets, are introduced at the start of the mixing cycle and are randomly scattered throughout the pellet mass. Some antioxidant particles adhere to the pellets via the static charge which builds on the pellet surfaces due to collision of the pellets; however, the dispersion achieved is very arbitrary and totally nonuniform, some of the pellets not receiving any antioxidant coating at all. Subsequent peroxide addition which later coats the particles uniformly partially conveys the antioxidant particles into the pellets, again randomly and completely nonuniformly. A further drawback of this process is that during the time interval required to agitate the polyethylene pellets and develop the surface charge to adhere antioxidant particles, the friction between pellets abrades the surface generating an excessive amount of polyethylene dust, commonly termed "fines," which is detrimental in later processing of the polyethylene into cable insulation.
In our invention, we have found a method to improve the degree of dispersion of nonmeltable (at processing temperatures) antioxidants to a state wherein all agglomeration associated with conventional processes described above is completely eliminated. Additionally, in the case of polyethylene mixing described in Method 3 above, all of the desirable features of the process such as manufacture directly from pellets, reduction of heat history and loss of antioxidant associated with the higher temperature mixing such as in Methods 1 and 2, is retained while at the same time, generation of fines is virtually eliminated also.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.